For a few months now, I've had a strange whistling noise that hasn't effected performance but I still want to know what it is. It's a low whistle tone that initiates when I let off on the throttle after revving the engine - I took apart the air box and couldn't find anything loose in there, which was my initial assumption. Oh, and the sound started after I got on it pretty hard one day. Any thoughts or suggestions are greatly appreciated, especially if they're simple. . . there might have been something stupid that I've overlooked (like normal lol).